77 
(Fig. 6, h.) has the joint below the division split up somewhat ir- 
regularly, and the branches have the two lower joints coalesced. 
— These cases of dichotomy, although worth mentioning, do not 
afford any special interest. 
Considerably more interest attaches to two specimens having 
each one pinnule developed into a true arm. In one of them 
it is the third pinnule on the outer side of the posterior arm, left 
anterior radius, which is developed as shown in Fig. 6, a. The 
proximal part has the character of a true genital pinnule, but from 
the 8th joint pinnules are developed regularly alternating as in a 
true arm, the joints of the main pinnule assuming a much more robust 
character than the normal pinnule joints, although not so robust 
as normal armjoints. The 15—16th joints form a syzygy and the 
third joint thereafter again is a syzygy. There are 6 pinnules to 
each side, all except the last one on the distal side having the 
genital organs strongly developed. After the 6th pinnule the main 
axis continues and ends as a usual pinnule. 
In the other specimen it is the third pinnule on the outer side 
of the anterior arm of the right anterior radius, which has devel¬ 
oped into an arm (Fig. 6, e). The four proximal joints have the char¬ 
acter of true pinnule joints, only somewhat more robust than usual. 
From the fifth joint pinnules are developed, alternating regularly, and 
the joints of the main axis assume the character of true armjoints, 
the 10—11 th forming a syzygy. The point of the arm is broken, 
the preserved rest carrying four pinnules on each side, all of which 
have the character of genital pinnules. The basal part of the main 
pinnule does not carry a genital organ, which was the case in the 
former instance. 
No other abnormalities occur in the two last mentioned specimens. 
It is a very noteworthy faet that in the arms developed from 
the oral pinnules no genital organs are found, while in those devel- 
oping from the genital pinnules all the pinnules, also the lower- 
most of them, have well developed genital organs. This is, of 
course, in accordance with the structural difference between oral 
and genital pinnules, the genital rhachis not sending a branch into 
the former, while the water vessel is found there normally, so that 
the pinnules on the arms developing from the oral pinnules can 
have the water vascular apparatus completely developed. 
